Re: [PATCH v2] mm: fix MAX_FOLIO_ORDER on powerpc configs with hugetlb

["David Hildenbrand (Red Hat)" <david@kernel.org>]

On 15.11.25 10:37, Christophe Leroy wrote:
> 
> 
> Le 14/11/2025 à 22:49, David Hildenbrand (Red Hat) a écrit :
>> In the past, CONFIG_ARCH_HAS_GIGANTIC_PAGE indicated that we support
>> runtime allocation of gigantic hugetlb folios. In the meantime it evolved
>> into a generic way for the architecture to state that it supports
>> gigantic hugetlb folios.
>>
>> In commit fae7d834c43c ("mm: add __dump_folio()") we started using
>> CONFIG_ARCH_HAS_GIGANTIC_PAGE to decide MAX_FOLIO_ORDER: whether we could
>> have folios larger than what the buddy can handle. In the context of
>> that commit, we started using MAX_FOLIO_ORDER to detect page corruptions
>> when dumping tail pages of folios. Before that commit, we assumed that
>> we cannot have folios larger than the highest buddy order, which was
>> obviously wrong.
>>
>> In commit 7b4f21f5e038 ("mm/hugetlb: check for unreasonable folio sizes
>> when registering hstate"), we used MAX_FOLIO_ORDER to detect
>> inconsistencies, and in fact, we found some now.
>>
>> Powerpc allows for configs that can allocate gigantic folio during boot
>> (not at runtime), that do not set CONFIG_ARCH_HAS_GIGANTIC_PAGE and can
>> exceed PUD_ORDER.
>>
>> To fix it, let's make powerpc select CONFIG_ARCH_HAS_GIGANTIC_PAGE with
>> hugetlb on powerpc, and increase the maximum folio size with hugetlb to 16
>> GiB on 64bit (possible on arm64 and powerpc) and 1 GiB on 32 bit (powerpc).
>> Note that on some powerpc configurations, whether we actually have gigantic
>> pages depends on the setting of CONFIG_ARCH_FORCE_MAX_ORDER, but there is
>> nothing really problematic about setting it unconditionally: we just try to
>> keep the value small so we can better detect problems in __dump_folio()
>> and inconsistencies around the expected largest folio in the system.
>>
>> Ideally, we'd have a better way to obtain the maximum hugetlb folio size
>> and detect ourselves whether we really end up with gigantic folios. Let's
>> defer bigger changes and fix the warnings first.
>>
>> While at it, handle gigantic DAX folios more clearly: DAX can only
>> end up creating gigantic folios with HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD.
>>
>> Add a new Kconfig option HAVE_GIGANTIC_FOLIOS to make both cases
>> clearer. In particular, worry about ARCH_HAS_GIGANTIC_PAGE only with
>> HUGETLB_PAGE.
>>
>> Note: with enabling CONFIG_ARCH_HAS_GIGANTIC_PAGE on powerpc, we will now
>> also allow for runtime allocations of folios in some more powerpc configs.
>> I don't think this is a problem, but if it is we could handle it through
>> __HAVE_ARCH_GIGANTIC_PAGE_RUNTIME_SUPPORTED.
> 
> Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> 
> Tested on powerpc 8xx with CONFIG_ARCH_FORCE_MAX_ORDER=8 instead of 9.
> It is now possible to add hugepages with the following command:
> 
> echo 4 > /sys/kernel/mm/hugepages/hugepages-8192kB/nr_hugepages
> 
> But only if CONFIG_CMA is set.
> 
> Tested-by: Christophe Leroy <christophe.leroy@csgroup.eu>

Thanks a lot for the review and test!

(thanks to the other reviewers obviously as well :) )

-- 
Cheers

David